The present invention generally relates to displaying of physiological data on patient monitors. More particularly, the present invention relates to a method and apparatus of correcting for phase error induced by down sampling of physiological data.
Patient physiological data, such as ECG data, is commonly displayed on patient monitors as a waveform suitable for review by medical care personnel. In order for medical care personnel to correctly assess the clinical import of the displayed information, it is highly desirable that the waveforms accurately reflect the measured physiological data regardless of the monitor type, pixel resolution, the size of the window in which the waveforms or other data are displayed or the speed at which the data is sampled. Generally, this has required some specific scale factor to be employed for each monitor type, pixel resolution capacity or display window size being employed.
One source of waveform distortion results from symmetrically plotting non-uniformly spaced selected data points in down-sampled data. When down-sampling data, features of the waveform in the original data may be lost or distorted by the down-sampling process. For example, the resolution of a standard cathode ray tube (CRT) monitor is not adequate for presenting full resolution ECG data. ECG data is typically received from the patient at a rate of about 480 Hertz (Hz)3. The ECG data is filtered for electrical noise and other extraneous electrical information, reducing the data rate to about 240 Hz and presented to the clinician using a standard scroll rate of 25 millimeters (mm) per second (sec). A scroll rate of 25 mm/sec is a standard within the health care industry. A typical 21xe2x80x3 CRT monitor displays a horizontal image size of approximately 406.4 mm with a horizontal screen resolution of 1280 pixels, or 3.15 pixels/mm. At a scroll rate of 25 mm/sec, each pixel represents approximately 12.7 milliseconds (ms) of data. Thus, if data is received at a 240 Hz rate, a data point is displayed on the CRT every 4.167 ms. This means that 3.05 data points will map to the same horizontal pixel location on the CRT. If the data is received at a different rate, e.g., 120 Hz (such as is the case with blood pressure waveforms) 1.52 data points will map to the same horizontal pixel location on the CRT. Plotting data as a waveform in this manner results in the plotting of different data points on the same pixel location. As a result, important physical features of the waveform representing the data may be lost.
Accordingly, the invention provides an interface for accurately displaying non-uniformly spaced selected data points in down-sampled data for a computer monitor display.
According to one aspect of the invention, a method and apparatus for synchronously plotting non-uniformly spaced selected data points in down-sampled data is provided. A data signal is received at a first rate and separated into at least one data window. Each data window has a predetermined number of data points having respective values. At least one of either a minimum and a maximum value of the data points are identified, thus identifying a position of the one of the minimum and maximum value relative to a reference. The data point is then displayed having the one of the minimum and maximum value at the position.
The invention also provides an apparatus having an input for receiving a data signal sampled at a first rate and means for converting the data signal to a second data signal sampled at a second lower rate and for displaying the second data signal on a video monitor.
It is an advantage of the invention to provide an interface for accurately displaying non-uniformly spaced selected data points in down-sampled data for a computer monitor display.
It is another advantage of the invention to provide a method and apparatus for synchronously plotting non-uniformly spaced selected data points in down-sampled data.